Sphingolipids are integral components of all eukaryotic cell membranes; many of them like ceramide, sphingosine, sphingosine-1 -phosphate (S-1-P) are also bioactive lipids regulating cellular functions ranging from apoptosis to angiogenesis. Our long-term goal is to comprehensively understand functions of enzymes that control normal sphingolipid metabolism and their integration into other pathways involved in cell growth, differentiation and signaling. To address our goal, we study enzymes of sphingolipid biosynthesis using Drosophila as a model system. This proposal is based on the observation that sphingolipid metabolism plays an important role in survival of photoreceptors. Our findings show that decreasing ceramide levels by genetic modulation of the sphingolipid biosynthetic pathway suppresses retinal degeneration in a set of phototransduction mutants. Recent studies in human patients show that mutations in enzymes of ceramide metabolism cause Retinitis Pigmentosa and Hereditary Sensory Neuropathy. Based on these findings, the focus of this project is to understand how ceramide metabolism contributes to maintenance of photoreceptor homeostasis and other cellular functions. We will elucidate in vivo functions of three enzymes that decrease ceramide levels - ceramidase that converts ceramide to sphingosine, ceramide kinase that converts ceramide to ceramide-1-phosphate and sphingosine kinase that converts sphingosine to S-1-P. While ceramide and sphingosine are pro-death, S-1-P is pro-growth. The relative level of these antagonistic metabolites, regulated by the biosynthetic enzymes, determines whether a cell survives or dies. The specific aims of the project are (1) study components downstream of ceramidase in suppression of retinal degeneration and elucidate ceramidase function using a mutant (2) generate ceramide kinase mutant and transgenic flies to study its functions (3) generate and analyze mutant and transgenic flies to define the role of sphingosine kinase in regulating levels of sphingolipid metabolites.